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Descriptions

The Devonian Simonson and Guilmette Formations provide examples of
Cordilleran miogeosynclinal inner and middle shelf carbonate deposition
that act in response to changes in eustasy, sedimentation, and
tectonics. The Simonson Dolomite consists of four members that
collectively record three distinct transgressive-regressive cycles.
The Coarse Crystalline dolomudstone lithofacies represents restricted
upper intertidal to supratidal sedimentation deposited on a distally
steepened carbonate ramp. Evolution of the Cordilleran shelf occurred
concurrent with deposition of the Lower Alternating cryptalgal
dolomudstone lithofacies. A depositional rimmed margin developed
recording lagoonal deposition, in the form of upward-shallowing
sequences, across the entire Middle Devonian shelf. A continued
relative sea-level rise is represented by the lower intertidal to
subtidal stromatoporoidal biostromal bank buildups of the Simonson
Brown Cliff Member. A relative sea-level fall is recorded by the
intertidal to supratidal Upper Alternating dolomud-wackestone
lithofacies.
Four lithofacies and nine subfacies are differentiated for the
Guilmette Formation, representing a broad spectrum of shallow subtidal
to intertidal environments. Thick buildups of primarily wackestone and
packstone lithofacies accumulated in response to a continued relative
sea-level rise. The extensive lateral growth of stromatoporoidal
biostromal bank buildups formed under open-marine conditions while
more restricted lagoonal environments dominated the inner shelf.
Acting in response to initial Antler orogenic foreland effects and in
concert with a continuing rise of sea-level, the Guilmette lithotope
was eliminated in the Late Devonian along with the Cordilleran
depositional rimmed margin.
Diagenetic fabric relationships observed in the Simonson and
Guilmette reveal a complex history beginning with the early-submarine
cementation of aragonite and/or high-Mg calcite. Neomorphic processes
followed and resulted in the stabilization to low-Mg calcite.
Diagenesis continued in the deep subsurface and included silica
cementation and replacement events that both predated and act
concurrently with pressure-solution stylolitization and
dolomitization. Late diagenetic silicification postdated both burial
dolomitization and secondary porosity evolution. Calcite pore and
fracture-filling cementation postdated all diagenetic events.
Diagenetic textures reflect deep-burial processes rather than the
original depositional environment. Reservoir qualities have been
enhanced by late-diagenetic dolomitization producing secondary
intercrystalline porosity within dolomitic Guilmette horizons and
especially in the Simonson Coarse Crystalline Member.